Generally speaking, oxygenated blood travels from the lungs to the left atrium by way of the pulmonary veins. The veins from the systemic circuit, the venae cavae and coronary sinus carry blood deficient in oxygen into the right atrium. The right ventricle takes blood received from the right atrium and sends it to the lungs, while the left ventricle takes blood received from the left atrium and sends it to the aorta.
The atrioventricular valves between respective ones of the atria and ventricles play important roles in the transport of blood through the body. The atrioventricular valves open during diastole, when the heart muscle relaxes, to allow blood to flow from the atria into the ventricles. The atrioventricular valves close during systole, when the heart muscle contracts, preventing the backflow of blood into the atria and allowing blood from the ventricles to be efficiently pumped into the lungs via the pulmonary artery and to the rest of the body via the aorta.
The mitral valve is the atrioventricular valve that controls blood flow from the left atrium into the left ventricle. The mitral valve is a bicuspid valve, describing the two cusps or leaflets that open and close the valve. The cusps or leaflets are attached to a muscular and fibrous ring around the orifice (mitral valve annulus) and their apices hang down into the left ventricle. When the ventricle fills with blood and begins to contract, the valve cusps or leaflets flow into position in the atrioventricular opening and are forced shut (coaptate) by the increasing pressure. To prevent the valve cusps or leaflets from turning into the left atrium and regurgitating blood, tendinous cords, the chordae tendineae, are attached to the free margins and ventricular surfaces of the cusps or leaflets. At the other ends, these cords are attached to one of a respective pair of papillary muscles projecting from the ventricular wall. By contracting, these muscles maintain the integrity of the valve during ventricular contraction or systole.
When the two cusps or leaflets of the mitral valve do not completely close, there is backflow, or regurgitation of blood. The backflow increases the pressure in the left atrium which leads to pulmonary hypertension and dilation of the heart which are the most common symptoms to congestive heart failure. A heart then has to work harder pumping blood for the body which can lead to heart damage. Incomplete closing of the mitral valve cusps or leaflets is common, occurring generally in about seven percent of the population. Conditions contributing to incomplete closure of the mitral valve cusps or leaflets include genetic defects, infections, coronary artery disease, myocardial infarction, or congestive heart failure. These conditions contribute to mitral valve regurgitation resulting from enlargement of the mitral valve annulus and/or movement of the papillary muscles away from the valve as a result of ventricular enlargement. When the annulus enlarges, the cusps or leaflets of the valve are no longer able to close (coaptate), because the distance between the two cusps or leaflets has increased too much for the cusps or leaflets to touch each other and thus close off blood flow to the left atrium during, for example, systole. Mitral valve regurgitation can also result as a secondary etiology due to the remodeling of a distorted left ventricle in ischemic heart disease. It is known that as the ventricle is remodeled, the papillary muscles can be displaced away from their natural position. This displacement alters the natural tethering of the cusps or leaflets and restricts the ability of the cusps or leaflets to close properly at the level of the annulus.
In general, most cases of mitral valve regurgitation are mild and the symptoms may be controlled with drugs. In more serious cases, the mitral valve can be repaired through a procedure known as annuloplasty, a surgical procedure in which a synthetic ring is placed around the valve annulus. Annuloplasty encourages aptation of the mitral valve cusps or leaflets by shrinking the size of the valve opening. In other instances, a faulty mitral valve must be surgically replaced with a new valve. These surgical repairs require the opening of the chest by sternotomy or at best through small incisions in the chest wall, heart lung bypass and stopping the heart beat. Further techniques under investigation include remodeling the adjacent coronary sinus and joining two middle edges of the cusps or leaflets where they should coaptate.
A second type of regurgitation occurs not necessarily by the enlargement of the mitral valve annulus but by the extending of a cusp or leaflet into the atrium during systole. A condition known as billowing occurs when the mitral valve cusps or leaflets do not meet well but instead get pushed up into the atrium. A condition known as prolapse occurs generally when a single cusp or leaflet extends into the atrium causing incomplete closure of the valve. A condition known as flail typically occurs when a tendon is ruptured and the corresponding cusp or leaflet extends into the atrium during systole.
Current mitral valve regurgitation may be diagnosed by Trans-Thoracic Echo (TTE) in many patients or Trans-Esophagael Echo (TEE). TEE tends to provide the most reliable and definitive structural and functional mitral valve information. Both TTE and TEE imaging devices are reusable.
TTE images the heart with a hand-held transducer from under the rib cage and between the ribs and thus has limited views of the mitral valve. TTE becomes less reliable in large or obese patients as the increased distance from the probe to the valve reduces image echo strength and resolution. Also, the imaging windows between the ribs become narrower as the probe is further away from the ribs in obese patients.
TEE images the heart from inside the esophagus (canal from the throat to the stomach) using an articulating probe and is relatively unaffected by patient size. However, it is very uncomfortable for a conscious patient and some patients cannot tolerate it while conscious. TEE is commonly used to check a surgical repair prior to closing the chest.
Another diagnostic technique is Intra-Cardiac Echo (ICE). One ICE is the ACUSON ACU-NAV™ System (10F) manufactured by Siemens Corporation. ICE is a one time use, array type articulating probe, placed in the right heart and, consequently, is relatively expensive and thus is not widely used at this time. It provides good views of the mitral valve, but may not have as good Doppler views as TTE or TEE.
The historical standard for diagnosing mitral valve regurgitation is angiographically observing the regurgitation of contrast injected into the left ventricle and is the basis of the common grading system for regurgitation (1+, 2+, etc.). It is widely recognized that the angiographic technique is not as reliable or as good an outcome predictor as the measurements of regurgitant volume and flow cross-section that can be made with the Doppler enhancements of modern echo systems.